The present invention relates to a method of making a preform for an ultratransparent or an amplifying optical fiber based on fluoride glass, e.g. of the ZBLAN or BIZYT type, for use in optical telecommunications.
Currently, such fibers have an attenuation rate of 0.65 dB/km over 110 meters for a signal having a wavelength of 1.55 .mu.m. The aim is to approach the ideal attenuation rate of 0.01 dB/km to 0.02 dB/km over distances of several tens, or even hundreds, of kilometers.
The high attenuation currently measured is attributed, in particular, to extrinsic defects that occur either during the preparation of the starting materials, or else while the glass that is to constitute the core or the cladding of the fiber is being made.
The usual methods of making the glass are based on the technique of a melt in a crucible.
Given the reactive nature of the fluorides used, making the glass in a crucible gives rise to two phenomena having adverse effects:
absorbant and diffusive impurities from the crucible are incorporated in the glass; and
heterogeneous nucleation resulting from chemical interaction between the glass and the walls of the crucible is stimulated, with large crystallites (characteristic size of about one micrometer) ultimately being formed in the glass, which crystallites are intense diffusion sources.
Such glass-making "defects" thus result in the optical fiber obtained having an attenuation rate that is very far removed from the ideal limits.
In order to avoid using a melt for manufacturing a preform for an optical fiber made of fluoride glass, consideration has been given to using the sol-gel technique of deposition inside a support tube. That technique is based on using carbon-containing precursors which are organic complexes based on .beta.-diketones of the metals making up the glass. Reference may be made to the article "sol-gel preparation of amorphous ZBLA heavy metal fluoride powders" by P. J. Melling and M. A. Thomson, J. Mater. Res. vol 5, No. 5 (1990) 1092 and D. R. Ulrich, NATO ASI, Sci, Ser. E123 (1987), 385.
That method does not achieve the desired improvement, in particular because of the problem of completely eliminating the carbon and the oxygen that come from the precursors.
Methods are also known that use chemical vapor deposition (CVD) inside a support tube, which methods also implement the same oxygen-and-carbon-containing precursors with the same difficulties of eliminating the carbon and the oxygen as in the above-mentioned method. The following documents describe such methods:
"Organometallic Chemical Vapor Deposition of ZrF.sub.4 -based fluoride glasses" K. Fujiura, Y. O kishi, S. Takahashi, Jpn. Appl. Phys. 28-1 (1989); and
"Preparation and properties of ZrF.sub.4 based fluoride glass films by plasma enhanced Chemical Vapor Deposition", K. Fujiura, Y. Nishida, K. Kobayashi and S. Takahashi, Jpn. J. Appl. phys. 30-8B (1991) L1498.